Such a solenoid valve is known, for example, from DE 20 2005 013 233 U1.
The use of a pilot valve has the advantage that only the pilot valve element has to be actuated to control the function of the main valve means. The forces needed for moving the pilot valve element are much smaller than the forces needed for moving the main valve element. Therefore, a pilot control solenoid valve can be used to control fluids under high pressure, for example carbon dioxide, without dramatically increasing the coil and yoke arrangement.
In some cases it is required that a solenoid valve has a large opening stroke, i. e. the solenoid valve has a low flow resistance in fully open state. This means that the main valve means has to be opened to a rather large extend to that the main valve element must be able to perform a large opening stroke. Consequently, the pilot valve element must be able to perform a similar large openings stroke. When the pilot valve is closed, the armature means has the largest distance to the yoke arrangement so that the magnetic forces which can attract the armature means are quite low. Therefore, when a large opening stroke is required, the coil and yoke arrangement must be increased to a considerable size to generate the necessary magnetic attraction forces.
Such a solenoid valve can be, for example, used in a multiejector. In this case it is required to generate maximum velocity of the controlled gas at the so called motive nozzle. This is done by minimizing the pressure losses. One contribution to the low pressure loss comes from a large diameter or large opening when the valve is open. A large diameter here means that the main valve element needs to move a significant part of, as a rule, at least ¼ of the diameter of an outlet bore. This means that there are low magnetic forces available because the magnetic forces vary over the distance from the yoke arrangement to the armature means.
Furthermore, when the solenoid valve is used in a CO2 system the pressure difference is significantly higher than for other refrigerants. In a CO2 system pressure difference of at least 50 bar is possible and can be significantly higher, for example 90 bar.
This means that the valve has to open with relative weak magnetic forces while being able to cope with a significantly higher pressure difference.